To Understand The Atom Is
To Understand Chemistry
Learning Objectives
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Describe the three particles in the atom
Define atomic number and mass number
Describe isotopes
Write symbols for elements
Determine the numbers of particles in any
atom from the element symbol
Determine average atomic mass from isotope
distribution
Identify important groups in periodic table
Atoms are not the
smallest thing
Growing evidence for the divisibility
of the indivisible
Electrostatics and electricity
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Static electricity was observed by
Thales (300 BC). Some
“charged” objects repel and
others attract
The voltaic cell (Volta, 18th
century) generated electrical
current from chemical reactions
Mechanical electrical generation
was achieved in 1825
The point: Atoms are neutral. If
indivisible, where do electrical
charges come from?
Faraday’s prescience
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“Although we know nothing of what an atom
is, we cannot resist forming some idea of a
small particle; and though we are in equal
ignorance of electricity, there is an immensity
of facts which justify us in believing that the
atoms of matter are associated with electrical
powers to which they owe their most striking
qualities, and amongst them their chemical
affinity.”
Ray of hope
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1858
Discovery of cathode rays by Julius Plucker
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Application of a large voltage across an evacuated
tube causes a current to flow. The current flow is
accompanied by radiation from the excited gas
molecules
How does the neutral and indivisible atom create a
charge?
Cathode rays are negatively charged particles
1897.J. Thomson demonstrates that cathode rays consist of negatively charged
particles. The first sighting of the electron: a particle much smaller than an atom.
The “oil drop” experiment gave a
measurement of electron charge
The Thomson model of the atom
"I regard the atom as containing a large number of smaller bodies
which I will call corpuscles, these corpuscles are equal to each
other.... In the normal atom, this assemblage of corpuscles
forms a system which is electrically neutral. Though the
individual corpuscles behave
like negative ions, yet when they are
assembled in a neutral atom the negative
effect is balanced by something which
causes the space through which the corpuscles are spread to act as
if it had a charge of positive electricity equal in amount to the
sum of the negative charges of the corpuscles…”
X-rays and atoms’ invisible rays
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1895
Wilhelm Conrad
Roentgen discovers Xrays while doing
experiments with
cathode rays
Radioactivity: the search for
invisible rays
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1896
Henri Becquerel discovered radioactivity,
which suggested that some atoms were capable
of decomposing to give smaller particles.
1903
Frederick Soddy and William Ramsey
demonstrated that uranium decayed to give
helium. Direct proof that atoms were
divisible.
1909 Ernest Rutherford’s gold foil
experiment
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The atom’s inner secrets exposed by its own
offspring
Hierarchy of discovery
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The path to knowledge occurs in steps, each
depending on a previous advance
The battery and electricity: Volta 1799
Cathode rays: Plucker 1858
X-rays: Roentgen 1895
Radioactivity: Becquerel 1896
The nucleus: Rutherford 1909
The nucleus
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Tiny
Incredibly dense – contains all the mass
of the atom
Positively charged
Contains protons (charged) and neutrons
(neutral) – not discovered until much
later
Summary of Atom Pictures
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Dalton: Indivisible atom
Thomson: Electrons
Rutherford: Nucleus
Atoms are mostly nothing
Comparison of subatomic particles
Mass
Particle
grams
amu
Charge
coulombs
e
Electron
9.1094 x 10-28 5.486 x 10-4
-1.6022 x 10-19
-1
Proton
1.6726 x 10-24 1.0073
+1.6022 x 10-19
+1
Neutron
1.6749 x 10-24 1.0087
0
0
Atoms are neutral:
# electrons = # protons
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The number of protons in the nucleus of an atom
determines the atomic number (Z) and indicates the
element's identity. For a neutral atom, the atomic
number also describes the number of electrons around
the nucleus.
Variations on the number of neutrons in the
nucleus give rise to different isotopes of the
same element.
1
1
H
2
1
H
3
1
H
Element notation: Atomic number
and mass number
Mass number:
number of protons
+ neutrons
13
6
C
Element symbol
Atomic number:
number of protons
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Counting particles:
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Number of electrons = number of protons = 6
Number of neutrons = mass number – atomic number (13 –
6 = 7)
Atomic mass
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Atoms are very small and so normal units of
mass are inconvenient for describing atomic
mass
Atomic Mass Unit
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Mass of one atom of carbon-12 = 12 amu
1 amu = 1.660 x 10-24 g
Similarity of atomic mass and mass
number
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Almost all mass is
concentrated in the
nucleons
Means mass
number (integer)
and atomic mass are
very similar
Isotopes cause disparity
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Carbon has two isotopes: C-12 and C-13
Atomic mass C-12 = 12 amu; C-13 = 13.0034
amu
Atomic mass C is weighted average of the
masses of the two isotopes (98.89 % C-12 +
1.11 % C-13
= 12.011 amu
What of the electrons?
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We now understand the atom to contain a tiny
positively charged massive nucleus surrounded
by a comparatively vast empty space
containing the electrons
When atoms combine the electrons must
interact
We need to understand the arrangement of
electrons in the atom
Rudiments of elements
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All of matter is made from elements, in combination with other elements or
alone.
There are some ninety naturally occurring – have “always” been.
More than twenty have been created artificially
Element names and symbols
Aluminum
Al
Chlorine
Cl
Manganese
Mn
Copper (cuprium)
Cu
Argon
Ar
Fluorine
F
Nitrogen
N
Iron (ferrum)
Fe
Barium
Ba
Helium
He
Oxygen
O
Lead (plumbum)
Pb
Boron
B
Hydrogen
H
Phosphorous
P
Mercury
(hydrargyrum)
Hg
Bromine
Br
Iodine
I
Silicon
Si
Potassium (karium)
K
Calcium
Ca
Lithium
Li
Sulfur
S
Silver (argentum)
Ag
Carbon
C
Magnesium
Mg
Zinc
Zn
Sodium (natrium)
Na
Elements organized – a triumph of
chemistry
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The periodic table of the elements is organized into 18 groups and 7
periods. Elements are represented by one- or two-letter symbols and are
arranged according to atomic number.
History of discovery
The Periodic Table: Groups and Periods
Groups: columns of
elements
Periods: rows of
elements
Elements in groups have similar
properties
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One of the eight main groups of elements, Group 1A
is known as the alkali metals group. Elements in this
group are highly reactive and form alkaline (basic)
solutions with water.
All groups have characteristic
properties
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One of the eight main groups of elements, Group 8A is known as the noble
gas group. Elements in this family have such a low reactivity that they were
formerly known as the inert gases.
Although their chemical properties are very uninteresting, their importance
to understanding chemistry cannot be understated
Group 7A – the halogens
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Reactive nonmetals
Fluorine and chlorine
are gases
Bromine liquid
Iodine solid